The present invention relates to a method and a device for improving the control behavior of an anti-lock brake system adapted for active braking intervention, more particularly for maintaining the driving stability and for improving vehicle steerability.
It is known in the art to extend the functions of an ABS system by employing the system for improving the driving stability or brake stability during cornering. Such a control management is effected during partial braking, i.e., during a braking operation where the ABS response thresholds are not reached. A stabilizing moment about the vertical axis of a vehicle is produced by specifically retarding the braking pressure increase on the bend-inward wheels, which is caused by application of the brakes, in comparison to the braking pressure on the bend-outward wheels (xe2x80x98Brake System and Slip Control System of the new BMW 7-series, ATZ 97 (1995), pages 8-15, and xe2x80x98Brake System and Slip Control Systems of the new BMW 5-seriesxe2x80x99, ATZ 98 (1996), pages 188-194). When no steering angle sensor is used, the information about the current steering angle is derived from the transverse acceleration of the vehicle, which, in turn, is calculated from the wheel sensor signals.
German patent application No. 39 39 069 discloses an automotive vehicle wherein a signal is produced upon the occurrence of a predetermined brake slip and/or lateral slip on the rear wheels without brake operation. There is provision of a cornering identification device, which identifies cornering and the type of the bend. When cornering is identified, the brake is not applied, and in the presence of longitudinal slip on the wheels, braking pressure is introduced on the rear axle, preferably, only on the brake of the bend-outward rear wheel. The objective is to increase the cornering force on the bend-outward rear wheel and to counteract veering of the vehicle.
German patent No. 34 13 738 (P5547) discloses an anti-lock control system (ABS) with a cornering identification circuit, which is also based on wheel slip measurement. For cornering identification, the slip values on the wheels of one vehicle side are added, compared to the slip sum of the wheels on the other vehicle side, and a cornering identification signal is generated as soon as the difference of the slip sums exceeds a predetermined limit value. During cornering identification, selection criteria such as xe2x80x98select-lowxe2x80x99 or xe2x80x98select-highxe2x80x99, according to which the pressure variation in the individual braking pressure control channels of this brake system is controlled, as well as limit values for the selection criteria becoming effective are varied. This way, the control is conformed to the different conditions prevailing during straight travel and during cornering.
Beyond a partial braking operation, stabilizing a vehicle by braking intervention is of course possible only if braking pressure can be generated in the brake system and/or in the wheel brakes even without application of the brake pedal or without operation of the braking pressure generator. Known brake systems with traction slip control (TCS) and/or driving stability control (DSC, ASMS, etc.) are capable of active braking intervention.
The prior art driving stability control systems are equipped with yaw rate sensors, steering angle sensors, etc., which are required in addition to the wheel speed sensors, and with sophisticated processing and monitoring systems. When yaw torques are detected, which jeopardize driving stability, evaluation of all measured values is followed by selectively controlling the wheel brakes to generate a balancing and stabilizing yaw torque about the vertical axis of the vehicle.
An object of the present invention is to improve the control behavior of an anti-lock brake system and to extend the control functions in a simple way and with at most little additional effort and structure in order to maintain the steerability and driving stability of the vehicle during cornering even when the brake is not applied.
It has been found that this object can be achieved in a very simple manner by the method of the present invention. Thus, the special features of the method of the present invention include that, when the following conditions, i.e.,
cornering situation
the vehicle decelerates although the brake is not applied, are simultaneously identified or satisfied, a critical slip condition is confirmed, which indicates veering of the vehicle or that a bend limit range is reached.
When these conditions are simultaneously found, a special control is started, which causes braking pressure delivery at least into the wheel brake of the bend-outward front wheel.
It has proved expedient in some cases to limit the delivery of braking pressure exclusively to the bend-outward front wheel. An asymmetrical braking pressure introduction into both front-wheel brakes is preferred in other cases. Principally, the higher braking pressure is required on the bend-outward front wheel to stabilize the vehicle, which tends to veer. Decelerating both front wheels is advantageous because the imminent instability is typically caused by a too high speed in the bend, which is not in conformity with requirements.
According to another preferred aspect of the present invention, the occurrence of a higher slip on the bend-outward rear wheel in comparison to the bend-outward front wheel is assessed as a criterion for that the critical slip condition is satisfied. This slip ratio indicates the xe2x80x98veeringxe2x80x99 of the vehicle or that the bend limit range is reached due to cornering at too high speed.
It is also possible to determine and analyze the instantaneously fastest vehicle wheel instead of scrutinizing the slip. When the bend-outward front wheel rotates at the highest speed, and this comparison is made on the basis of filtered wheel signals, the critical condition has occurred, i.e., a higher slip on the bend-outward rear wheel compared to the bend-outward front wheel.
The stabilizing introduction of braking pressure into the bend-outward front wheel or asymmetrically into both front wheels is expediently effected by one single braking pressure introduction pulse of e.g. 200-300 msec or by a corresponding pulse train. On the other hand, the special control and, thus, the pressure introduction can be continued until the critical slip condition no longer prevails or, in other words, until the bend-outward wheels signal approximately the same amount of slip.
An algorithm for identifying cornering and some further favorable embodiments of the present invention, including a device for implementing the method of the present invention, are also described.